It is well-known that alcohol can be recovered by subjecting a fermented mash to a series of distillation steps. The resulting alcohol has heretofore been predominately used for the production of alcoholic beverages.
Recently, however, the so-called "energy crisis" has created significant interest in the conversion of biomass to fuel grade alcohol which can be blended with no-lead gasoline to produce "Gasohol" fuel. Fuel grade alcohol may be produced from any material containing cellulose, starch or sugar. For example, the United States has an abundant supply of corn and wheat which are considered primary feedstocks.
In conventional processing the grain feedstock is milled and then diluted with both a recycle and fresh process water. This mixture is then heated or "cooked" by direct steam injection to 212.degree.-300.degree. F. to initiate liquifaction and to sterilize the mixture. Enzymes are added to the solution both before and after cooking to further break down the complex starch molecules and to convert starch to sugar.
Subsequently, the solution is cooled to about 85.degree.-90.degree. F. and fed to a fermentation zone. Yeast is added and the sugar is converted to alcohol and carbon dioxide. From this batch-wise fermentation process an alcohol concentration of between about 7 and 15% is achieved. 95 volume percent alcohol is recovered by conventional distillation and absolute alcohol (greater than 99.5%) can be obtained by azeotropic distillation.
A dilute aqueous solution containing soluble and insoluble organic matter, fibers, and ash from the grains is obtained from the fermentation zone. This solid slurry is processed through an evaporation and drying zone.
Evaporation is accomplished in a multi-effect unit having at least two regions operating at successively lower pressures and temperatures. Typically, the highest pressure region (the first region) operates at a pressure of between about 15 and 45 psia and the last and lowest pressure region exhibits a pressure between about 1.0 and 20 psia. Overhead vapor from the first region is supplied to the second region, etc. Vapor from the last region may either be condensed or compressed for recycling to the first region.
The last process step involves drying to produce dried distillers grain which is valuable animal feed. Conventional practice utilizes indirect steam at a pressure of 100 to 250 psia or hot flue gas to provide heat for drying. The vapors produced in the dryer are typically vented into the atmosphere. The dryers are typically operated by using a "sweep air" to carry the water vapor from the drying grains.